Egg shells return calcium and phosphorus to the soil. If you want to start plants from seeds, a very handy technique is to wash the inside of an eggshell, put some soil in each half, and plant a seed in each soil cup. When it starts to sprout you can plant the whole thing. If you want to enrich the soil with eggshells wash the inside before you crush them, then work into the soil near the plant.

Thank you for this info, I hate those shell creatures, I always use salt on them, but I can't get them all and now I know how to prevent them from eating all the leaves on my orange tree, you would think the stickers would deter them, but evidently not.